This invention relates to preparing carbon fibrils.
Carbon fibrils are carbon microfibers having diameters less than 500 nanometers. They may be prepared by contacting a metal-containing catalyst with a carbon-containing gas at elevated temperatures.
In a first aspect, the invention features a fibril aggregate that includes a multiplicity of carbon fibrils whose longitudinal axes have substantially the same relative orientation, each of the fibrils characterized as having graphitic layers that are substantially parallel to its longitudinal axis and being free of a continuous thermal carbon overcoat (i.e. pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare the fibrils). One aspect of substantial parallelism is that the projection of the graphitic layers on the fibril""s longitudinal axis extends for a relatively long distance in terms of the external diameter of the fibril (e.g., at least two fibril diameters, preferably at least five diameters), as described in Snyder et al., U.S. Ser. No. 149,573 filed Jan. 28, 1988 refiled as continuation application Ser. No. 494,894, filed Mar. 13, 1990, refiled as continuation application Ser. No. 694,244, filed May 1, 1991; and entitled xe2x80x9cCarbon Fibrilsxe2x80x9d which is assigned to the same assignee as the present application and hereby incorporated by reference. Carbon fibrils having substantially parallel graphitic layers are also described in Tennent, U.S. Pat. No. 4,663,230 (xe2x80x9cCarbon Fibrils, Method for Producing Same and Compositions Containing Samexe2x80x9d), Tennent et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986 refiled as continuation application Ser. No. 593,319 filed Oct. 1, 1990, now U.S. Pat. No. 5,165,909, issued Nov. 24, 1992. (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Compositions Containing Samexe2x80x9d), Tennent et al., U.S. Ser. No. 871,675 filed Jun. 6, 1986 refiled as continuation application Ser. No. 492,365 filed Mar. 9, 1990, now U.S. Pat. No. 5,171,560, issued Dec. 15, 1992; (xe2x80x9cNovel Carbon Fibrils, Method for Producing Same and Encapsulated Catalystxe2x80x9d), Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 refiled as continuation application Ser. No. 746,065, filed Aug. 12, 1991, refiled as continuation application Ser. No. 08/284,855, filed Aug. 2, 1994; (xe2x80x9cFibrilsxe2x80x9d), and McCarthy et al., U.S. Ser. No. 351,967 filed May 15, 1989 refiled as continuation application Ser. No. 823,021, refiled as continuation application Ser. No. 117,873, refiled as continuation application Ser. No. 08/329,774, filed Oct. 27, 1994. (xe2x80x9cSurface Treatment of Carbon Microfibersxe2x80x9d), all of which are assigned to the same assignee as the present application and are hereby incorporated by reference.
In preferred embodiments, the diameters of at least 90% (and, more preferably, substantially all) of the fibrils in the aggregate have diameters between 3.5 and 75 nanometers, inclusive. Similarly, at least 90% (and, more preferably, substantially all) of the individual fibrils in the aggregate have a length to diameter ratio of at least 5. The diameter of the aggregate preferably is between 0.05 and 50 xcexcm, inclusive, and the length preferably is between 0.1 and 1000 xcexcm, inclusive.
In a second aspect, the invention features a process for preparing an aggregate of carbon fibrils by. contacting a particulate metal catalyst deposited on a support having one or more readily cleavable planar surfaces and a surface area of at least 1 m2/g with a carbon-containing gas in a reactor at reaction conditions including temperature sufficient to produce the aggregate.
In preferred embodiments, the support is a metal oxide, e.g., xcex3-alumina or magnesia, both of which are in the form of aggregates of tabular, prismatic, or platelet crystals. Preferred catalysts include iron. They may further include at least one element chosen from Group V (e.g., vanadium), VI (e.g., molybdenum, tungsten, or chromium), VII (e.g., manganese), or the lanthanides (e.g., cerium). Also preferred are catalysts that include cobalt, nickel, manganese, or a combination of copper and zinc. The catalysts may be prepared using either aqueous or non-aqueous solvents.
Preferred reaction temperatures are between 400 and 850xc2x0 C., more preferably between 600 and 750xc2x0 C. Preferred aggregates are those aggregates described above in which the longitudinal axes of the fibrils making up the aggregate all have substantially the same relative orientation.
The invention also features a particulate, carbon fibril-forming, metal catalyst deposited on a support having one or more readily cleavable planar surfaces and a surface area of at least 1 m2/g. Preferred catalyst and support materials are those described above.
The invention provides a process for preparing fibril aggregates in which the texture of the aggregate is controlled by the choice of catalyst support. Using supports having one or more readily cleavable planar surfaces produces fibril aggregates having the appearance of combed yarn in which the individual fibrils are straight to slightly bent or kinked. Aggregates having loose, open mat textures in which the individual fibrils are straight to slightly bent or kinked may also be produced. These aggregates are readily dispersed, making them useful in composite fabrication where uniform properties throughout the structure are desired. The substantial linearity of the individual fibril strands also makes the aggregates useful in EMI shielding and electrical applications, e.g., the devices described in Friend et al., U.S. Ser. No. 08/284,738, filed Aug. 2, 1994, which is a continuation of 692,819, which is a continuation of 413,1844, filed Sep. 28, 1989; entitled xe2x80x9cBatteryxe2x80x9d and Friend et al., U.S. Ser. No. 602,446, filed Oct. 23, 1990, now U.S. Pat. No. 5,110,693, issued May 5, 1992, which is a continuation of Ser. No. 413,838, filed Sep. 28, 1989, now abandoned entitled xe2x80x9cElectrochemical Cellxe2x80x9d, both of which were filed on the same day as the present application, assigned to the same assignee as the present application, and hereby incorporated by reference in their entirety.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.
We now describe the structure and preparation of preferred fibril aggregates.
Structure
Preferred fibril aggregates consist of bundles of straight to slightly bent or kinked carbon fibrils in which the individual fibrils have substantially the same relative orientation, e.g., the longitudinal axis of each fibril (despite individual bends or kinks) extends in the same direction as that of the surrounding fibrils in the bundle. This arrangement of individual fibrils gives the aggregates the appearance of combed yarn, in contrast to aggregates such as those produced according to the process described in the aforementioned Snyder et al. application, U.S. Ser. No. 149,573, in which the fibrils are randomly entangled with each other to form tightly entangled balls of fibrils resembling bird nests.
The carbon fibrils within each fibril aggregate preferably have diameters between about 3.5 and 75 nanometers, length to diameter ratios of at least 5, and graphitic layers that are substantially parallel to the longitudinal fibril axis, and are also substantially free of a continuous thermal carbon overcoat, as described in Tennent, U.S. Pat. No. 4,663,230; Tennent et al., U.S. Ser. No. 871,676; Tennent et al., U.S. Ser. No. 871,675; Snyder et al., U.S. Ser. No. 149,573; and Mandeville et al., U.S. Ser. No. 285,817. The aggregates may also be treated to introduce oxygen-containing functional groups onto the surface of individual fibrils, as described in McCarthy et al., U.S. Ser. No. 351,967. Within each fibril aggregate, the diameters and length to diameter ratios of the individual fibrils are essentially uniform.
A second type of fibril aggregate consists of straight to slightly bent or kinked fibrils which are loosely entangled with each other to form an, xe2x80x9copen matxe2x80x9d structure. The degree of entanglement is greater than observed in the combed yarn aggregates (in which the individual fibrils have substantially the same relative orientation) but less than that of the tightly entangled fibril balls formed according to the process described in Snyder et al., U.S. Ser. No. 149,573.
Preparation
In general, both the combed yarn and open mat aggregates are prepared by contacting an iron or iron-containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 m2/g with a carbon-containing gas in a reactor at 400-850xc2x0 C. using the procedures described in the aforementioned Tennent patent and Tennent, Snyder, and Mandeville applications.
Preferred support materials include xcex3-alumina or magnesia in the form of aggregates of tabular, prismatic, or platelet crystals. Such material is commercially available, e.g., from Strem Chemicals (in the case of xcex3-alumina) and Alfa Inorganics (in the case of magnesia). The xcex3-alumina supports yield primarily combed yarn aggregates, while the magnesia supports yield primarily open mat aggregates. In contrast, the use of supports consisting of spherical particles or aggregates lacking cleavable planar surfaces (e.g., supports made of Degussa fumed alumina as described in the aforementioned Snyder et al. application) leads primarily to tightly entangled fibril balls.
While not wishing to be limited to any particular theory, it is believed that the readily cleavable planar surfaces of the support allow the fibrils to assist each other as they grow, creating a xe2x80x9cneighbor effectxe2x80x9d that, in the case of the xcex3-alumina support, leads to a combed yarn fibril aggregate in which the individual fibrils have the same relative orientation. Spherical supports, on the other hand, lack this effect, leading to tightly entangled balls of randomly oriented fibrils. The magnesia support, although having readily cleavable planar surfaces, yields primarily lightly entangled, open mat fibril aggregates because it breaks apart more readily than the xcex3-alumina support during fibril growth, resulting in aggregates that are less ordered than the combed yarn aggregates but more ordered than the tightly entangled fibril balls. The oxide precursors used to generate the metal catalyst particles also affect the tendency of the support to break apart. The more readily the oxide and support can form a mixed oxide at the interface between them, the more likely the support is to break apart.